Study calls for nitrate, water watch

Field demonstrations on vegetable farms in the southern Santa Clara Valley have shown that growers can gain efficiency with a closer watch — and more complete records — on their irrigation and fertility programs.

Marc Buchanan, a Scotts Valley soil fertility consultant, headed a two-year project that set out to help growers with in-field nitrate testing on drip and sprinkler irrigated vegetables to economize on fertilizer use.

Although Santa Clara Valley growers are reluctant to risk yields by cutting back on nitrogen and irrigation inputs, regulations forcing it are approaching.

Agricultural fertilization, along with residential septic systems, animal enclosures, and other sources, is charged with contributing excessive nitrate levels in surface and underground waters.

Further, the Central Coast Regional Water Quality Control Board is developing daily load limits for nitrates and other nutrients in the Llagas Basin, a part of the Pajaro River Watershed. The region of 330,000 acres has about 7,200 acres in various crops, including 35 to 40 vegetable crops.

Five key growers, with farming operations from Gilroy to Morgan Hill, cooperated in the project funded by the California Department of Food and Agriculture's Fertilizer Research and Education Program (FREP), which is dedicated to cost-effective fertilizer use and minimal environmental impacts.

Partner in the project is the Santa Clara Valley Water District's Nitrate Management Program, which operates a mobile lab program for irrigation system evaluation.

Nitrogen quick test

In conclusions presented at a FREP Conference in Tulare recently, Buchanan said the nitrogen quick test was again demonstrated as an effective means of fertility management, including evaluating residual nitrate, scheduling side- and top-dress fertigation, and assessing problem areas in the field.

However, Buchanan said, the structure of farming businesses dispersed over a large area tend to impede more monitoring for nitrates. One grower, for instance, has 20 fields across the basin, five irrigators, and a ranch manager who spends much of his time driving on Highway 101 between fields.

As another example, a local ranch manager wants to adopt changes, such as reducing applied nitrogen, but does not have approval from a headquarters superior to spend less than a certain amount allocated for nitrogen.

Yet another grower has a file of petiole sample results taken by a commercial fertilizer company but is unable to interpret and apply them.

Not only do growers vary as to abilities, they have varying interest in making changes. “One of the biggest issues is records and evaluations,” said Buchanan.

“Among several cooperating growers, only one, a recent college graduate, could remember what he did the year before in terms of fertilization or any evaluation of his crop.”

Management ‘snapshot’

Buchanan said his aim in working with cooperating growers and in public workshops is provide a “snapshot” to help them realize that nitrogen management of their fields involves water, soil, uniformity and scheduling of irrigation, plus disease at times, and proper interpretation of the whole.

He selected cooperating ranches that operated both sprinkler and drip irrigation systems. A “technical vacuum,” he said, exists in the area, after growers doing their own experimentation in shifting to drip. Some, even when they converted to drip where they could meter-in precise amounts, continued to apply large amounts of preplant nitrogen fertilizers in “a sort of hybrid fertility practice.”

Several growers consider high fertility a tool to advance their crop during periods when soil temperatures are low. “In the cooler 2000 season, they used nitrogen to jump-start the crop, and we were able to establish for those growers that the added nitrogen is really not doing them a favor.”

Buchanan also set out to demonstrate the utility of the work on the nitrate “quick test” by Tom Hartz, University of California, Davis vegetable crops specialist, and Richard Smith, Monterey County farm advisor. Used in the field, the method uses colormetric strips reveal nitrate in the soil, and its accuracy is comparable to laboratory results.

In one example of a “hybrid” fertility program, the grower typically applied 70 pounds of preplant nitrogen for peppers on buried drip. “After two irrigations the plants are set, and you see a salt band on the furrows and on the bed shoulders,” he said.

This, he continued, suggests that much of the preplant nitrogen was being forced away from roots and to the soil surface.

One grower used the nitrate test method to assess residual soil nitrate in residue from a preceding crop and a cover crop and then tap it rather than simply applying more nitrogen for the new crop.

Another started monitoring for soil nitrate and saved applying 150 pounds of nitrogen through proper timing and methods of application.

Mixing and mismatching of high- and low-flow drip tapes robbed some growers of full potential of their systems. In one example, tensiometers showed that the portion on low-flow tape did get the uniform wetting to take up all the nitrogen available.

Irrigation scheduling was frequently more of a problem than distribution uniformity, particularly for the growth stages of bell peppers, among cooperators. “In the 2001 season,” Buchanan reported, “we found that four of five fields were under-irrigated prior to fruit bulking stage and then over-irrigated prior to first harvest and for the rest of the season.”

Further, some fields on coarse soils had in-season leaching, and growers typically overcorrected with increased fertilization.

Although the original project is completed, Buchanan said the Santa Clara Valley Water District has indicated interest in seeking funding for a follow-up program to continue workshops, demonstrations, and evaluations.

Marc Buchanan is a researcher-consultant specializing in soil fertility and management of solid and water wastes and based at Scotts Valley, Calif. His company, Buchanan Associates, was established in 1995 and its service area includes San Mateo, Santa Clara, Santa Cruz, and San Benito counties.

From 1990 to 1995 he was a professor at the University of California, Santa Cruz. He holds a doctorate in soil fertility and soil management from North Carolina State University and a master's from UC, Santa Cruz.